Deployment systems, such as wireline, for deploying tools in oil wells are widely used. In operation, a wireline deployment system runs a wireline cable from a surface vessel down to a subsea wellhead. The subsea wellhead comprises a BOP package on the seabed, a lubricator system attached to the BOP and a stuffing box through which the wireline cable passes to access the lubricator system. Tools are connected or disconnected from the end of the wireline cable in the lubricator system, and once ready for downhole operations, the well is opened up and the wireline, complete with tools, is lowered in.
The combined weight of the tools is generally sufficient to allow for progression of the tool down the well. In deviated wells a tractor maybe used to assist in progression of the tools down the well. In either case, the weight of the tools and/or the action of the tractor applies a downward pull force to the wireline.
This arrangement works well at depths of 500 meters or less. At these depths, the wireline cable will enter the stuffing box in a substantially vertical orientation, that is when the cable enters the well it is substantially aligned with the entrance to the well. However at depths greater than 500 meters the effects of currents can drag the wireline cable into a more curved or caternary shape, with the result that the angle of entry into the well becomes closer to horizontal.
In this orientation there is a greater frictional resistance between the cable and the subsea apparatus. Furthermore, the drag of the current can apply an upward pull on the wireline, countering the effects of the downward pull force applied to the wireline by gravity and/or the action of the tractor. In extreme cases the drag-induced upward pull and/or the frictional resistance loading applied to the wireline is so great that it overcomes the downward pull force preventing the wireline tools from descending into the well.